Hi,
I have been getting this error message quite frequently. Somethings it fixes itself and the times I am stuck.
See screen shot.
It seems to fix itself if I restart Visual Studio, but why does it happen?
Hi,
This will occur when only the file is open and not the solution/project. It is the project file which holds the references to the other libraries which you are referencing in your code, UnityEngine.dll for example and many others.
What is the version number of Visual Studio you are using? There was an issue with 15.9.2 which was not reloading the projects within Visual Studio after changes were made in Unity which also led to these issues.